Exposure control method and apparatus for an image pickup apparatus

ABSTRACT

The invention concerns with an exposure control mechanism for use in an image pickup apparatus in which a diaphragm aperture formed by a plurality of diaphragm blades moving straightforward in opposite directions is covered by an ND filter, and intends to prevent deterioration of image quality caused by diffraction even in an image pickup device having a small picture size and a short pixel pitch. The image pickup apparatus comprises an exposure control mechanism for adjusting the quantity of light flux entering a shooting lens system. The exposure control mechanism comprises a diaphragm made up of diaphragm blades movable on a plane perpendicular to an optical axis in opposite directions to define a diaphragm aperture, and an ND filter made up of at least two ND filter elements having different transmittances. When the diaphragm blades are displaced from an aperture open state in a direction to restrict the quantity of transmitting light, an aperture area is restricted by the diaphragm blades from the open state to a predetermined aperture area, and thereafter the ND filter is advanced into the diaphragm aperture successively from one of the ND filter elements having the highest transmittance while the predetermined aperture area is maintained.

RELATED APPLICATION

This application is a continuation of application Ser. No. 09/357,892,filed on Jul. 21, 1999, which is now U.S. Pat. No. 6,771,315.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an exposure control mechanism for usein an image pickup apparatus in which a diaphragm aperture formed by aplurality of diaphragm blades moving straightforward in oppositedirections is covered by an ND filter, and more particularly to thetechnique of suppressing deterioration of image quality caused bydiffraction even in an image pickup device having a small picture sizeand a short pixel pitch.

2. Description of the Related Art

In image pickup apparatuses such as video cameras, an exposure controlmechanism comprising two diaphragm blades moved on a straight line inopposite directions for reduction of size, weight and cost have becomemore commonly used instead of the so-called “iris diaphragm” wherein aplurality of diaphragm blades are rotated about an optical axis toadjust the aperture size.

However, if the aperture size becomes too small when a subject isbright, there occur such problems that image quality is deteriorated dueto diffraction and smudges are brought into an image due to an increaseof the depth of focus.

In view of those problems, a mechanism has been proposed in which an NDfilter is affixed to one of diaphragm blades in such a manner as toproject into a cutout which is formed in the diaphragm blade to definethe aperture size, so that the aperture is avoided from becomingextremely small.

FIG. 14 shows one example of an exposure control mechanism for use inconventional image pickup apparatuses.

An exposure control mechanism a comprises two diaphragm blades b, c anda drive means d for driving the diaphragm blades b, c.

One (front) diaphragm blade b has a cutout e formed at a lower end fordefining the aperture size. Two guided slits f, f extending verticallyare formed in the diaphragm blade b at a position near a right edgethereof in vertically spaced relation. Also, a guided slit: g extendingvertically is formed in the diaphragm blade b at a position near a leftedge thereof.

It is to be noted that directions U, D, L, R, F and B indicated byarrows in the drawings, including FIG. 14, represent the upward,downward, leftward, rightward, forward and backward directions,respectively.

A connecting slot h being elongate horizontally is formed in thediaphragm blade b at a position just above the upper right guided slitf.

Guide pins provided on a housing (not shown), which has formed therein alight passing hole, are slidably engaged in the guided slits f, f and g,respectively. The diaphragm blade b is thereby supported by the housingin a vertically slidable manner.

The other (rear) diaphragm blade c has a cutout i formed at an upper endfor defining the aperture size, and an ND filter j is attached to thediaphragm blade c so as to cover a lower end area of the aperture sizedefining cutout i. Two guided slits k, k extending vertically are formedin the diaphragm blade c at a position near a left edge thereof invertically spaced relation. Also, a guided slit 1 extending verticallyis formed in the diaphragm blade b at a position near a right edgethereof. Incidentally, the ND filter j has transmittance of 10%.

Furthermore, a connecting slot m being elongate horizontally is formedin the diaphragm blade c at a position just above the upper left guidedslit k.

Guide pins provided on the housing (not shown) are slidably engaged inthe guided slits k, k and l, respectively. The diaphragm blade c isthereby supported by the housing in a vertically slidable manner.

The drive means d comprises a drive motor n attached to an upper portionof the housing (not shown), and an operating arm o fixed to a rotaryshaft of the drive motor n.

The operating arm o extends substantially in the right-and-leftdirection, and is fixed at its central portion to the rotary shaft ofthe drive motor n. Also, connecting pins p, p are projected respectivelyfrom right and left ends of the operating arm o.

The connecting pin p at the right end of the operating arm o is slidablyengaged in the connecting slot h of the diaphragm blade b, and theconnecting pin p at the left end of the operating arm o is slidablyengaged in the connecting slot m of the diaphragm blade c.

Accordingly, when the operating arm o is rotated by energizing the drivemotor n, the connecting pins p, p are moved in opposite directions,whereupon the diaphragm blades b, c coupled to the connecting pins p, pare moved vertically in opposite directions. As a result, a diaphragmaperture q defined by the aperture size defining cutouts e, i of the twodiaphragm blades b, c is changed.

FIGS. 15 a to 15 f show a manner in which the ND filter j covers thediaphragm aperture q when the diaphragm aperture q is gradually narrowedfrom an open state (FIG. 15 a) to a small aperture state (FIG. 15 f) bymoving the diaphragm blades b, c of the exposure control mechanism a.

FIG. 16 shows values of an MTF (modulation transfer function) dependingon various sizes of the diaphragm aperture q indicated in FIGS. 15 a to15 f. Here, the MTF value means a diffraction limit value of the whiteMTF value determined by calculating, based on wave optics, thecapability in the vertical direction (line image in the horizontaldirection) at spatial frequency corresponding to the TV resolution ofabout 260 lines. Also, the dimension of the ND filter j is decided sothat the diaphragm aperture q has a size corresponding to F 5.6 at themoment when the diaphragm aperture q is entirely covered by the NDfilter j (see FIG. 15 e). The MTF value at that moment is 0.73.

Specifically, the MTF value means a diffraction limit value of the whiteMTF value determined by calculating, based on wave optics, thecapability in the vertical direction (line image in the horizontaldirection) that is evaluated by the fact that the effect of diffractionappears significantly in the states of FIGS. 15 a to 15 f, in view ofspatial frequency of 48 lines/mm corresponding to the TV resolution ofabout 260 lines, i.e., frequency representing image quality in a motionvideo camera comprising an image pickup device wherein the picturediagonal length is 4.5 mm, a pixel pitch is about 5 μm, and the numberof effective pixels is 380,000.

Accordingly, deterioration of image quality is regarded as being allowedif the MTF value is not less than a predetermined value. The MTF value=0.65 has been employed, by way of example, as an allowable limit valuein the past. Note that the MTF value is not an absolute value, but arelative value used for determining whether deterioration of imagequality is in the allowable range.

In the case of conventional image pickup devices in which the picturediagonal length is 4.5 mm, as shown by a solid line in FIG. 16, when thediaphragm blades b, c are moved to gradually narrow the diaphragmaperture q, the MTF value is also gradually reduced, and takes a minimumvalue in the state shown in FIG. 16 d, i.e., at the aperture size d.Then, the MTF value increases again and takes a maximum value in thestate shown in FIG. 16 e, i.e., at the aperture size e. Thereafter, theMTF value decreases again.

The reason why the MTF value takes a minimum value at the aperture sized is that a vacant space area surrounded by the diaphragm blade b andthe ND filter j serves as like a small aperture to develop diffraction,and image quality is deteriorated in an intermediate aperture state.

When the diaphragm blades b, c are further moved to gradually narrow thediaphragm aperture q, the MTF value increases again and takes a maximumvalue at the aperture size e. This is because until the diaphragmaperture q changes from the aperture size d to the aperture size e atwhich the diaphragm aperture q is completely covered by the ND filter j,the effect of diffraction is gradually reduced so that the MTF valueincreases. When the diaphragm aperture q is further narrowed from theaperture size e, the MTF value decreases again due to the effect ofdiffraction.

Taking into account such changes of the MTF value, it has been customarythat the transmittance of the ND filter j is designed to keep the MTFvalue not less than 0.65 in all the sates from the open aperture a tothe small aperture f.

Recently, in image pickup apparatuses, there has been a tendency toreduce the picture size of an image pickup device. A reduction in thepicture size of the image pickup device decreases the pixel pitch andincreases the effect of diffraction, thus making it hard to obtainsatisfactory image quality. While conventional image pickup devices hada picture diagonal length of 4.5 mm, for example, the picture diagonallength has been recently reduced to 2.25 mm. The spatial frequencycorresponding to the TV resolution of about 260 lines is 48 lines/mm forthe picture diagonal length of 4.5 mm, and 96 lines/mm for the picturediagonal length of 2.25 mm. If the picture diagonal length of an imagepickup device is changed to 4.5 mm with the pixel pitch remainedunchanged, this case corresponds to frequency representing image qualityin a still-video camera comprising 1.5 millions pixels.

When an image pickup device having a picture diagonal length of 2.25 mmis employed in combination with the conventional exposure controlmechanism a, the spatial frequency is doubled and therefore the effectof diffraction is remarkably increased, thus giving rise to a problemthat image quality is deteriorated.

More specifically, a broken line in FIG. 16 shows a curve of the MTFvalue resulted when the conventional exposure control mechanism a iscombined with such a small image pickup device. As seen, the MTF valueis reduced down below 0.65 in a state in which the diaphragm aperture qis narrowed from the aperture size b to some extent. This means that theabove combination is not practicable. In other words, the conventionalexposure control mechanism a has a problem of being not adaptable fordownsizing of the image pickup device.

SUMMARY OF THE INVENTION

In consideration of the problems as set forth above, the presentinvention concerns with an exposure control mechanism for use in animage pickup apparatus in which a diaphragm aperture formed by aplurality of diaphragm blades moving straightforward in oppositedirections is covered by an ND filter, and intends to preventdeterioration of image quality caused by diffraction even in an imagepickup device having a small picture size and a short pixel pitch.

To achieve the above object, the present invention provides an imagepickup-apparatus comprising an exposure control mechanism for adjustingthe quantity of light flux entering a shooting lens system, the exposurecontrol mechanism comprising a diaphragm made up of diaphragm bladesmovable on a plane perpendicular to an optical axis in oppositedirections to define a diaphragm aperture, and an ND filter made up ofat least two ND filter elements having different transmittances, whereinwhen the diaphragm blades are displaced from an aperture open state in adirection to restrict the quantity of transmitting light, an aperturearea is restricted by the diaphragm blades from the open state to apredetermined aperture area, and thereafter the ND filter is advancedinto the diaphragm aperture successively from one of the ND filterelements having the highest transmittance while the predeterminedaperture area is maintained.

In the image pickup apparatus of the present invention, the exposurecontrol mechanism includes the ND filter made up of at least two NDfilter elements having different transmittances, and the ND filter isadvanced into the diaphragm aperture successively from one of the NDfilter elements having the highest transmittance. Therefore, an imagepickup device having a small picture size and a short pixel pitch can beemployed with less deterioration of image quality.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of an exposure control mechanism,showing a first embodiment of the present invention along with FIGS. 2and 3;

FIGS. 2A, 2S and 2L are schematic views showing shapes of a diaphragmaperture of the exposure control mechanism, in which FIG. 2A representsan open aperture state, FIG. 2S represents a state having apredetermined aperture size, and FIG. 2L represents an intermediateaperture state;

FIGS. 3M to 3S′ are schematic views representing states in which filterelements of an ND filter are advanced to the diaphragm aperturesuccessively in the descending order from one having the highesttransmittance in the intermediate aperture state of the exposure controlmechanism, and FIG. 3T is a schematic view representing a state in whichthe diaphragm aperture is narrowed from the state of FIG. 3S′;

FIG. 4 is a graph showing the relationship between an MTF value and anaperture shape defined by diaphragm blades and the ND filter of theexposure control mechanism;

FIG. 5 is a block diagram of the image pickup apparatus for explainingan algorithm for controlling the exposure control mechanism in the firstembodiment of the image pickup apparatus of the present invention alongwith FIGS. 6 to 8;

FIG. 6 is a flowchart showing a process of calculating an amount to becontrolled for exposure control in a CPU.

FIG. 7 shows a manner of distributing an amount to be controlled in thenext cycle when the amount to be controlled in the next cycle is withinthe control range of a diaphragm.

FIG. 8 shows a manner of distributing an amount to be controlled in thenext cycle when the amount to be controlled in the next cycle is beyondthe control range of the diaphragm.

FIG. 9 is an exploded perspective view of an exposure control mechanism,showing a second embodiment of the present invention along with FIGS. 10and 12;

FIG. 10 is a front view of a principal part of the exposure controlmechanism shown in FIG. 9;

FIG. 11 is a graph showing the relationship between a rotational angleof a rotating plate of the exposure control mechanism shown in FIG. 9and respective strokes through which each diaphragm blade and an NDfilter holding member are moved;

FIG. 12 is an exploded perspective view showing a first modification ofthe ND filter.

FIG. 13 is an exploded perspective view showing a second modification ofthe ND filter.

FIG. 14 is an exploded perspective view showing an exposure controlmechanism for use in conventional image pickup apparatus along withFIGS. 15 and 16.

FIGS. 15 a to 15 f are schematic views showing shapes of a diaphragmaperture successively from an open aperture state (FIG. 15 a) to a smallaperture state (FIG. 15 f).

FIG. 16 is a graph showing the relationship between an MTF value and ashape of the diaphragm aperture.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Details of an image pickup apparatus of the present invention will bedescribed below in connection with embodiments shown in the accompanyingdrawings.

FIGS. 1 to 4 show a first embodiment of the image pickup apparatus ofthe present invention. An exposure control mechanism 1 for use in theimage pickup apparatus according to the first embodiment differs fromthe above-described conventional exposure control mechanism a in that anND filter holding member for holding an ND filter and a moving means formoving the ND filter holding member are provided. Also, the exposurecontrol mechanism 1 for use in the image pickup apparatus according tothe first embodiment is applied to an image pickup device which has apicture diagonal length of 2.25 mm (spatial frequency of 96 lines/mm).

The exposure control mechanism 1 comprises two diaphragm blades 2, 3disposed to be vertically movable in opposite directions, a diaphragmblade driving mechanism 4 for moving the diaphragm blades 2, 3, an NDfilter holding member 5 to which an ND filter (described later) isattached, and an ND filter driving mechanism 6 for moving the ND filterholding member 5, etc. Note that directions U, D, L, R, F and Bindicated by arrows in the drawings, including FIG. 1, represent theupward, downward, leftward, rightward, forward and backward directions,respectively.

The diaphragm blades 2, 3 and the ND filter holding member 5 are eachformed of a resin film having relatively high stiffness. One diaphragmblade 3 is positioned between the other diaphragm blade 2 and the NDfilter holding member 5. These three components are arranged in ashooting lens system such that the diaphragm blade 2 is positioned onthe object side and the ND filter holding member 5 is positioned on theimage side.

The diaphragm blades 2, 3 and the ND filter holding member 5 are, thoughnow shown, vertically slidably disposed in a box-like rectangularhousing which is flat in the back-and-forth direction and is elongate inthe vertical direction. Circular light passing holes are formed in wallsof the housing.

The diaphragm blade 2 is substantially J-shaped as viewed from the backside, and a substantially semicircular large cutout 7 for defining theaperture size is formed at an upper end of a lower portion of thediaphragm blade 2. A lower end area 7 a of the aperture size definingcutout 7 is formed to have a substantially triangular shape.

Two guided slits 8, 8 extending vertically are formed in the diaphragmblade 2 at a position near a left edge thereof in vertically spacedrelation. Also, a guided slit 9 extending vertically is formed in thediaphragm blade 2 at a position near a right edge thereof. A connectingslot 10 being elongate horizontally is formed in the diaphragm blade 2at a position just above the upper left guided slit 8.

Two support pins provided on the left side of the housing (not shown)are slidably engaged in the left guided slits 8, 8, and a support pinprovided on the housing at a lower right position is slidably engaged inthe right guided slit 9, respectively. The diaphragm blade 2 is therebysupported by the housing in a vertically movable manner.

A substantially semicircular cutout 11 for defining the aperture size isformed at a lower end of the diaphragm blade 3 positioned between thediaphragm blade 2 and the ND filter holding member 5 in the direction ofan optical axis. An upper end area 11 a of the aperture size definingcutout 11 is formed to have a substantially triangular shape.

Two guided slits 12, 12 extending vertically are formed in the diaphragmblade 3 at a position near a right edge thereof in vertically spacedrelation. Also, a guided slit 13 extending vertically is formed in thediaphragm blade 3 at a position near a left edge thereof. A connectingslot 14 being elongate horizontally is formed in the diaphragm blade 3at a position just above the upper right guided slit 12.

Two support pins provided on the right side of the housing (not shown)are slidably engaged in the right guided slits 12, 12, and a support pinprovided on the housing at a lower left position is slidably engaged inthe left guided slit 13, respectively. The diaphragm blade 3 is therebysupported by the housing in a vertically movable manner.

Further, the diaphragm blades 2, 3 are vertically movable in oppositedirections, and an opening defined by the aperture size defining cutouts7, 11 overlapping with each other serves a diaphragm aperture 15. Thesize of the diaphragm aperture 15 is, as described later, changed by thediaphragm blade driving mechanism 4.

The ND filter holding member 5 has a cutout 16 formed in its centralportion to have a substantially U-shape being open upward as viewed fromthe front. The cutout 16 has a horizontal width substantially equal toor slightly greater than the horizontal width of the aperture sizedefining cutouts 7, 11, and an ND filter 17 is disposed in the cutout 16so as to cover the same. Also, in the ND filter holding member 5, aguided slit 18 is formed at a position near a right edge of the NDfilter 17, and a guided slit 19 is formed at a position near a left edgeof the ND filter 17. A connecting slot 20 being elongate horizontally isformed in the ND filter holding member 5 at a position just below theleft guided slit 19.

Two support pins provided on the right side of the housing (not shown)are slidably engaged in the right guided slit 18, and two support pinsprovided on the left side of the housing are slidably engaged in theleft guided slit 19, respectively. The ND filter holding member 5 isthereby supported by the housing in a vertically movable manner.

The ND filter 17 is made up of four filter elements 21 a, 21 b, 21 c and21 d which have different transmittances and are arranged side by sidein the vertical direction. The filter element 21 a positioned at the tophas the highest transmittance, and the transmittances of the otherfilter elements are gradually reduced toward the bottom one. The filterelements 21 a, 21 b, 21 c and 21 d of the ND filter 17 are formed bydepositing thin films on one transparent plate material so as to providedifferent transmittances by vapor deposition or any other suitablemeans.

More specifically, the filter element 21 a positioned at the top(hereinafter referred to as a “first filter element”) has transmittanceof 40%, the filter element 21 b positioned at the second level from thetop (hereinafter referred to as a “second filter element”) hastransmittance of 16%, the filter element 21 c positioned at the thirdlevel from the top (hereinafter referred to as a “third filter element”)has transmittance of 6.3%, and the filter element 21 d positioned at thebottom. (hereinafter referred to as a “fourth filter element”) hastransmittance of 2.5.

Further, the sizes of the filter elements in the vertical direction(i.e., the vertical widths of the filter elements) are selected asfollows. The upper three filter elements 21 a, 21 b and 21 c are formedto have the same vertical width β and to satisfy the relationship ofβ=α/2 wherein a is the vertical size of the diaphragm aperture 15 thatis resulted when the aperture is adjusted to a predetermined size. Also,the relationship between a vertical width γ of the fourth filter element21 d and the vertical width β of the other filter elements 21 a, 21 band 21 c is set to satisfy at least γ≧2β. In this first embodiment, γ=3βis selected.

A manner of deciding the predetermined aperture size α and thetransmittances of the filter elements 21 a, 21 b, 21 c and 21 d will nowbe described. It is to be noted that the exposure control mechanism 1according to the present invention intends, when applied to an imagepickup device having a picture diagonal length of 2.25 mm, to maintainimage quality comparable to that obtained when the exposure controlmechanism a described above in connection with the related art isapplied to an image pickup device having a picture diagonal length of4.5 mm, and therefore to keep the MTF value not less than 0.65 for thatpurpose.

First, for an image pickup device having a picture diagonal length of2.25 mm, the MTF value depending on the aperture size, resulted when thediaphragm aperture 15 is gradually narrowed without the ND filter, ismeasured and drawn in the form of a graph (indicated by a one-dot-chainline in FIG. 16). Then, an aperture size (see FIG. 2S), at which the MTFvalue takes 0.73, is determined. This aperture size gives the aforesaidpredetermined aperture size α.

The MTF value at the predetermined aperture size α corresponds to themaximum value in the graph of the MTF value obtained by the exposurecontrol mechanism a described above in connection with the related art(i.e., the graph indicated by the solid line in FIG. 16). The reason fordeciding the predetermined aperture size α in such a way is to attainsuch a result that a graph of the MTF value obtained using the ND filter17 by the exposure control mechanism 1 according to this embodiment isvaried within the range between the above MTF value (0.73) and the MTFvalue (0.65) at a lower allowable limit.

Next, the transmittance of the fourth filter element 21 d is decided sothat a transmitted light quantity ratio resulted from covering, by theND filter element having the lowest transmittance (i.e., the fourthfilter element 21 d), the entirety of the aperture size (correspondingto a state of FIG. 3T) at which the MTF value takes 0.65 when thediaphragm aperture 15 is further gradually narrowed without the NDfilter, becomes equal to the transmitted light quantity ratio resultedby the conventional exposure control mechanism a when the diaphragmaperture is minimized (i.e., in the state of FIG. 15 f with the NDfilter j having transmittance of 10%). In the case of the exposurecontrol mechanism 1 according to this embodiment, the transmittance ofthe fourth filter element 21 d thus decided is 2.5%. Here, the term“transmitted light quantity ratio” means a ratio of the quantity oftransmitted light when the aperture is open., to the quantity oftransmitted light when the aperture is narrowed to a certain size.

The transmittances of the other filter elements 21 a, 21 b and 21 c areonly required to satisfy such a relation that the transmittance of onefilter element is higher than the transmittances of the filter elementspositioned below the one. In view of the above, the transmittances ofthe other filter elements are designed to have the above-mentionedvalues.

Additionally, when taking a measure for suppressing diffraction occurredin the intermediate aperture state, it is effective to avoid a vacantspace area 28 (see FIG. 2L), which is surrounded by the aperture sizedefining cutout 11 of the diaphragm blade 3 and the first filter element21 a, from functioning as a small aperture. To this end, the filterelement having the highest transmittance (i.e., the first filter element21 a) is preferably designed to have transmittance as high as possible.

The diaphragm blade driving mechanism 4 of the exposure controlmechanism 1 comprises a motor 22 disposed in an upper portion of theexposure control mechanism 1, a rotating arm 23 driven by the motor 22,etc. The rotating arm 23 is attached to a rotary shaft of the motor 22.

The rotating arm 23 is fixed at its central portion to the rotary shaftof the motor 22, and small connecting pins 24 a, 24 b are projectedforward respectively from left and right ends of the rotating arm 23.The connecting pins 24 a, 24 b are positioned such that the distancesfrom the rotary shaft of the motor 22 to the connecting pins 24 a, 24 bare equal to each other.

The connecting pin 24 a at the left end of the rotating arm 23 isslidably engaged in the connecting slot 10 of the diaphragm blade 2, andthe connecting pin 24 b at the right end of the rotating arm 23 isslidably engaged in the connecting slot 14 of the diaphragm blade 3.

Accordingly, when the rotating arm 23 is rotated, the connecting pins 24a, 24 b are displaced vertically in opposite directions, whereupon thediaphragm blades 2, 3 are moved vertically in opposite directions. Atthis time, the diaphragm blades 2, 3 are moved vertically in oppositedirections through the same displacements, i.e., at the same speed.

With the vertical movements of the diaphragm blades 2, 3 in oppositedirections, the size of the opening defined by the aperture sizedefining cutouts 7, 11 overlapping with each other, i.e., the size ofthe diaphragm aperture 15, is changed as follows. The diaphragm aperture15 is minimized (corresponding to the small aperture state) when thediaphragm blade 2 is positioned at an upper end of the movable rangethereof and the diaphragm blade 3 is positioned at a lower end of themovable range thereof, and the diaphragm aperture 15 is maximized(corresponding to the open aperture state) when the diaphragm blade 2 ispositioned at a lower end of the movable range thereof and the diaphragmblade 3 is positioned at an upper end of the movable range thereof.Incidentally, the diaphragm aperture 15 in the open aperture state isnot defined by the aperture size defining cutouts 7, 11 overlapping witheach other, but has the same size as that of the light passing holeformed in the housing (not shown) of the exposure control mechanism 1.

The ND filter driving mechanism 6 of the exposure control mechanism 1comprises a motor 25 disposed in a lower portion of the exposure controlmechanism 1, a rotating arm 26 driven by the motor 25, etc. The rotatingarm 26 is attached to a rotary shaft of the motor 25.

The rotating arm 26 is fixed at its one base end to the rotary shaft ofthe motor 25, and a small connecting pin 27 is projected forward fromthe other distal end of the rotating arm 26.

The connecting pin 27 is slidably engaged in the connecting slot 20 ofthe ND filter holding member 5. Accordingly, when the rotating arm 26 isrotated, the ND filter holding member 5 is vertically moved.

The diaphragm blade driving mechanism 4 and the ND filter drivingmechanism 6 are driven, as described below, for forming the diaphragmaperture 15 and deciding the position of the ND filter 17 relative tothe diaphragm aperture 15.

First, the diaphragm blade driving mechanism 4 is driven to narrow thediaphragm aperture 15 from the open aperture state (FIG. 2A) to thestate (see FIG. 2S) having the predetermined aperture size α. Thediaphragm aperture 15 is then held in the predetermined aperture state.

Next, the ND filter driving mechanism 6 is driven to advance the NDfilter 17 into the diaphragm aperture 15 having the predeterminedaperture size α, starting from the filter element having the highesttransmittance (i.e., the first filter element 21 a) in a successivemanner (see FIGS. 2S, 2L, and 3M to 3T). The advance of the ND filter 17into the diaphragm aperture 15 is started immediately before thediaphragm aperture 15 takes the predetermined aperture size α. Thereason is to produce the so-called dead zone in which the quantity ofpassing light is not changed regardless of driving of the exposurecontrol mechanism 1. The presence of the dead zone makes it easy tocarry out various control of an optical system in an apparatus includingthe exposure control mechanism 1, e.g., a camera.

Subsequently, the diaphragm blade driving mechanism 4 is driven again tofurther narrow the diaphragm aperture 15 from the state (see FIG. 3S′)in which the diaphragm aperture 15 having the predetermined aperturesize α is covered by the fourth filter element 21 d only.

FIG. 4 is a graph showing the relationship between the respectivestates, which depend on changes in size of the diaphragm aperture 15 andchanges in movement of the ND filter at the predetermined aperture size,and MTF values corresponding to the respective states.

More specifically, as the diaphragm aperture 15 is narrowed from theopen aperture state A to the predetermined aperture size α(see FIG. 2S),the MTF value is gradually reduced and takes 0.73 in the state S. Then,as the ND filter 17 is advanced into the diaphragm aperture 15 havingthe predetermined aperture size α, the MTF value is further reduced andtakes a minimum value in the state (see FIG. 2L) in which the ND filter17 is advanced to occupy a lower ¾ area of the diaphragm aperture 15. Atthis time, the MTF value is slightly higher than 0.65. The state of FIG.2L corresponds to the state in which the vertical width β of the firstfilter element 21 a is entirely positioned in the diaphragm aperture 15and an upper ½ area (β/2) of the second filter element 21 b ispositioned in the diaphragm aperture 15. In the state of FIG. 2L, thevacant space area 28 is in the form of a small flat triangle. However,since a difference in transmittance between the vacant space area 28 andthe first filter element 21 a is smaller than conventional,deterioration of image quality caused by diffraction is alleviated.

The reason why the MTF value takes a minimum value when the diaphragmaperture 15 is in the state of FIG. 2L is that the vacant space area 28surrounded by the aperture size defining cutout 11 of the diaphragmblade 3 and the first filter element 21 a serves as like a smallaperture to develop diffraction, and image quality is somewhatdeteriorated.

When the ND filter 17 is further raised to such an extent that theentire vertical width β of the first filter element 21 a and the entirevertical width β of the second filter-element 21 b are both positionedin the diaphragm aperture 15, the MTF value takes a maximum value whichis slightly smaller than 0.73 (see FIG. 3M).

In this way, as the ND filter 17 is raised, two or three of the filterelements 21 a, 21 b, 21 c and 21 d are positioned in the diaphragmaperture 15, and the MTF value takes a minimum value when a smallaperture is formed between the filter element 21 having the highesttransmittance (i.e., the filter element 21 positioned at the top in thediaphragm aperture 15) or the vacant space area 28 and the aperture sizedefining cutout 11 of the diaphragm blade 3 (see FIGS. 2L, 3N, 3P and3R). Also, between one small aperture state and the next small aperturestate, the diaphragm aperture 15 is in a state other than the smallaperture state, and therefore the MTF value increases and takes amaximum value (see FIGS. 3M, 3O and 3Q).

When the diaphragm blade driving mechanism 4 is driven again from thestate (see FIG. 3S′) in which the diaphragm aperture 15 having thepredetermined aperture size α is covered by the fourth filter element 21d only, the MTF value is gradually reduced, and the diaphragm bladedriving mechanism 4 is stopped when reaching the state (see FIG. 3T) inwhich the MTF value takes 0.65.

Thus, an any of the states ranging from the state (see FIG. 2A) in whichthe diaphragm aperture 15 of the exposure control mechanism 1 is fullyopen to the state (see FIG. 3S′) in which the predetermined aperturesize α is covered by the fourth filter element 21 d only, as well as thestate (see FIG. 3T) in which the diaphragm aperture 15 of the exposurecontrol mechanism 1 is further narrowed from the predetermined aperturesize α, the MTF value takes a value not less than 0.65, i.e., anallowable value.

In the exposure control mechanism 1 described above, since the ND filter17 is advanced into the diaphragm aperture 15 while the diaphragmaperture 15 is fixedly held at the predetermined aperture size α,various control of the optical system can be performed with ease.

Additionally, in the image pickup apparatus of the present invention,the exposure control mechanism is not limited to the illustrated onewherein the ND filter is advanced into the diaphragm aperture while thediaphragm aperture is fixedly held at the predetermined aperture size.For example, the diaphragm aperture may be gradually narrowed whilecausing the ND filter to be advanced into the diaphragm aperture at ahigher speed than the narrowing speed of the diaphragm aperture.

An algorithm for control of the exposure control mechanism 1 accordingto the first embodiment will be described below with reference to FIGS.4 to 8.

In an image pickup apparatus 50, as shown in FIG. 5, a subject image isfocused on an image pickup device (CCD) 52 through a shooting lenssystem 51. Between the shooting lens system 51 and the CCD 52, there aredisposed the diaphragm blade 2, the diaphragm blade 3 and the ND filter17 which constitute the exposure control mechanism 1. These componentscooperatively adjust the quantity of light introduced to the CCD 52.

A video signal converted into an electric signal by the CCD 52 isfurther converted into a digital signal by an A/D converter 53. Thedigital signal is sent to a camera signal processing unit 54 in which aluminance signal component of the video signal is detected to determinethe brightness of the subject.

A value of the luminance signal detected in the camera signal processingunit 54 is sent to a CPU 55 which calculates amounts to be controlled bythe diaphragm blades 2, 3 and the ND filter 17 of the exposure controlmechanism 1. Control signals representing the amounts to be controlledare sent to a diaphragm blade driving circuit 56 and an ND filterdriving circuit 57 which operate the diaphragm blade driving mechanism 4and the ND filter driving mechanism 6, respectively, thereby adjustingthe quantity of exposure light.

Also, the video signal supplied to the camera signal processing unit 54is recorded in a recording medium 59 through a recorded signalprocessing unit 58. The recording medium 59 may be, e.g., a film-likerecording medium such as a silver salt film, a tape-like recordingmedium such as a video tape, a disk-like recording medium such as afloppy disk, an optical disk, a magneto-optical disk and a hard disk, ora semiconductor recording medium such as a detachable or stationarysolid state memory.

A process executed in the CPU 55 for calculating the amounts to becontrolled by the diaphragm blades 2, 3 and the ND filter 17 will bedescribed below.

As shown in FIG. 6, the value of the luminance signal detected in thecamera signal processing unit 54 is first read. The CPU 55 previouslystores a target value as a reference for the brightness of the subject,and an error amount is given by a ratio of the target value to thedetected value (see Formula 1 in FIG. 6). The error amount represents anerror amount regarding the amounts by which the diaphragm blades 2, 3and the ND filter 17 are controlled at present. Therefore, an amount 60to be controlled in the next cycle by the diaphragm blades 2, 3 and theND filter 17 is expressed by Formula 2 in FIG. 6.

Then, a control range of the diaphragm blades 2, 3 is set. In otherwords, this step decides how far the diaphragm aperture 15 is maximallynarrowed from a reference state in which the diaphragm blades 2, 3 areopen (see a range A in FIG. 7). The range A is decided based on MTF dataof the shooting lens system 50 so that image quality is not deterioratedwithin the range A.

Subsequently, a control range of the ND filter 17 (see a range B in FIG.7) is decided. This step is to decide the position at which advance ofthe ND filter 17 is started.

The amount 60 to be controlled in the next cycle is then distributedbetween the diaphragm blades 2, 3 and the ND filter 17. A distributingmanner in this step will be described with reference to FIGS. 7 and 8.

FIG. 7 shows the distributing manner when the amount 60 to be controlledin the next cycle is within the control range A achievable by thediaphragm blades 2, 3. Since the amount to be controlled is distributedwith priority given to the diaphragm blades 2, 3 as indicated by 61, theamount to be controlled by the ND filter 17 is zero. FIG. 8 shows thedistributing manner when the amount 60 to be controlled in the nextcycle is beyond the control range A achievable by the diaphragm blades2, 3. The amount 61 to be controlled by the diaphragm blades 2 reaches amaximum value, and a deficiency in comparison with the amount 60 to becontrolled in the next cycle is distributed as the amount 62 to becontrolled by the ND filter 17.

FIGS. 9 to 11 show a second embodiment of the image pickup apparatus ofthe present invention.

The second embodiment differs from the first embodiment in that twodriving mechanisms of the exposure control mechanism, i.e., one for thediaphragm blades 2, 3 (the diaphragm blade driving mechanism 4) and theother for the ND filter holding member 5 (the ND filter drivingmechanism 6), are integrated to one driving mechanism. The drawings showonly a principal part, and the following description will be made ofonly the different points between both the embodiments. A description ofother parts is omitted here while similar components in the drawings tothose of the image pickup apparatus according to the first embodimentare denoted by the same numerals. Also, the size of the diaphragmaperture and the position of the ND filter are changed in a like mannerto those in the exposure control mechanism 1 described above inconnection with the first embodiment.

A driving mechanism 29 for diaphragm blades 2A, 3A and an ND filterholding member 5A of an exposure control mechanism 1A comprises a motor30, a rotating plate 31 driven by the motor 30, etc.

Cam grooves 32, 33 are formed respectively in lower end portions of thediaphragm blades 2A, 3A, and a cam groove 34 is formed respectively in alower end portion of the ND filter holding member 5A.

The rotating plate 31 is substantially in the form of a disc, and hasconnecting pins 35 a, 35 b and 35 c projecting forward from threepredetermined positions on the rotating plate. These connecting pins 35a, 35 b and 35 c are provided to position on a concentric circle aboutthe center of rotation of the rotating plate 31. The connecting pins 35a, 35 b are disposed at the positions circumferentially spaced throughan included angle of 180 degrees about the center of rotation of therotating plate 31. In a state in which a line interconnecting the twoconnecting pins 35 a, 35 b lies almost horizontally, the connecting pin35 c is disposed at the position below the line and slightly nearer tothe connecting pin 35 a.

The connecting pin 35 a is slidably engaged in the cam groove 32 of thediaphragm blade 2A, the connecting pin 35 b is slidably engaged in thecam groove 33 of the diaphragm blade 3A, and the connecting pin 35 c isslidably engaged in the cam groove 33 of the ND filter holding member5A, respectively.

In a state in which the cam grooves 32, 33 of the diaphragm blades 2A,3A are engaged respectively with the connecting pins 35 a, 35 b of therotating plate 31, the cam grooves 32, 33 are located in pointsymmetrical relation about the center of rotation of the rotating plate31. More specifically, as shown in FIG. 10, portions 32 a, 33 a of thecam grooves 32, 33 except for opposite ends thereof are formed into anarc shape about the center of rotation of the rotating plate 31. Endportions 32 b, 33 b of the cam grooves 32, 33 in a direction opposite tothe clockwise direction (hereinafter referred to as “counterclockwiseend portions”) are formed to displace radially outward as they extend inthe counterclockwise direction, and end portions 32 c, 33 c of the camgrooves 32, 33 in the clockwise direction (hereinafter referred to as“clockwise end portions”) are formed to displace radially inward as theyextend in the clockwise direction (see FIG. 10).

The positional relationship between the connecting pins 35 a, 35 b andthe diaphragm aperture is set such that in a state in which the rotatingplate 31 is slightly rotated and the line interconnecting the twoconnecting pins 35 a, 35 b is also slightly rotated in the clockwisedirection from the horizontal position (see FIG. 10), the diaphragmaperture 15 is in the open aperture state (see FIG. 2A). In this state,the connecting pins 35 a, 35 b are positioned respectively in thecounterclockwise end portions 32 b, 33 b of the cam grooves 32, 33.

The cam groove 34 of the ND filter holding member 5A is formed into abow shape being convex upward but relatively flat. When the diaphragmaperture 15 is in the open aperture state, i.e., in the state of FIG.10, the cam groove 34 is located at a position slightly offset leftwardfrom a lower end of the rotating plate 31. Also, in the state of FIG.10, the connecting pin 35 c is positioned at a right end 34 a of the camgroove 34.

In the above arrangement, when the rotating plate 31 is rotatedclockwise in FIG. 10, the diaphragm blade 3A and the ND filter holdingmember 5A are moved up, whereas the diaphragm blade 2A is moved down,thus narrowing the size of the diaphragm aperture 15.

Then, when the connecting pins 35 a, 35 b are engaged respectively inthe arc-shaped portions 32 a, 33 a of the cam grooves 32, 33, thevertical movements of the diaphragm blades 2A, 3A are stopped. At thistime, the diaphragm aperture 15 takes the predetermined aperture size α(see FIG. 2S).

On the other hand, since the cam groove 34 engaging with the connectingpin 35 c is not arc-shaped, the ND filter holding member 5A continues tomove up with the rotation of the rotating plate 31, whereby the NDfilter 17 advances into the diaphragm aperture 15 having thepredetermined aperture size α. As the ND filter 17 advances into thediaphragm aperture 15 having the predetermined aperture size α, the MTFvalue is changed in the same manner as in the exposure control mechanism1 according to the first embodiment (see FIGS. 2S to 3S′).

Almost at the same time when the diaphragm aperture 15 having thepredetermined aperture size α is entirely covered by the fourth filterelement 21 d (see FIG. 3S′), the connecting pins 35 a, 35 b are engagedrespectively in the clockwise end portions 32 c, 33 c of the cam grooves32, 33. With the further rotation of the rotating plate 31, therefore,the diaphragm blade 3A starts to move up again, whereas the diaphragmblade 2A starts to move down again, thus narrowing the size of thediaphragm aperture 15 covered by the fourth filter element 21 d.Finally, when the connecting pins 35 a, 35 b and 35 c are positionedrespectively at the clockwise end portions 32 c, 33 c and a left end 34c of the cam grooves 32, 33 and 34, the motor 30 is stopped and themovements of the diaphragm blades 2A, 3A and the ND filter holdingmember 5A are also stopped.

FIG. 11 is a graph showing the relationship between a rotational angleof the rotating plate 31 and respective strokes through which thediaphragm blades 2A, 3A and the ND filter holding member 5A are moved.

With the exposure control mechanism 1A, since the cam grooves 32, 33 and34 are formed respectively in the diaphragm blades 2A, 3A and the NDfilter holding member 5A, the two diaphragm blades 2A, 3A and the NDfilter holding member 5A can be simultaneously moved by one drivingmechanism 29 over the predetermined ranges. Of course, by appropriatelychanging the shapes of the cam grooves 32, 33 and 34, the movements ofthe diaphragm blades 2A, 3A and the ND filter holding member 5A can becontrolled in a desired manner with ease.

FIG. 12 shows a first modification of the ND filter 17. An ND filter 36of this modification is formed by placing four filter plates 37 a, 37 b,37 c and 37 d, which have the same transmittance but are different invertical width, in adjacently overlapped relation in the back-and-forthdirection. The backmost filter plate 37 a has the largest verticalwidth, and the vertical widths of the other filter plates are graduallyreduced toward the frontmost filter plate 37 d. Further, those filterplates 37 a, 37 b, 37 c and 37 d are arranged such that their loweredges are flush with one another (see FIG. 12).

The difference in vertical width between the adjacent filter plates 37is set to β. In other words, a portion 38 a of the ND filter 36 formedby the filter plate 37 a alone, a portion 38 b of the ND filter 36 inwhich the two filter plates 37 a, 37 b are overlapped with each other,and a portion 38 c of the ND filter 36 in which the three filter plates37 a, 37 b and 37 c are overlapped with one another, each have avertical width β. Also, a portion 38 d of the ND filter 36 in which thefour filter plates 37 a, 37 b, 37 c and 37 d are overlapped with oneanother has a vertical width 3β. Thus the portions 38 a, 38 b, 38 c and38 d have respectively the same dimensions as the filter elements 21 a,21 b, 21 c and 21 d of the ND filter 17 in the exposure controlmechanism 1 according to the first embodiment.

The ND filter 36 is made up of the filter plates 37 a, 37 b, 37 c and 37d bonded to each other in close contact fashion. The portion 38 a of theND filter 36 formed by the single filter plate 37 a has transmittance of40%, the portion 38 b formed by the two overlapping filter plates 37 a,37 b has transmittance of 16% (40%×40%), the portion 38 c formed by thethree overlapping filter plates 37 a, 37 b and 37 c has transmittance of6.4% (40%×40%×40%), and the portion 38 d formed by the four overlappingfilter plates 37 a, 37 b, 37 c and 37 d has transmittance of 2.56%(40%×40%×40%×40%). Thus the portions 38 a, 38 b, 38 c and 38 d also haverespectively almost the same transmittances as the filter elements 21 a,21 b, 21 c and 21 d of the ND filter 17 in the exposure controlmechanism 1 according to the first embodiment.

With the ND filter 36 of this modification, an ND filter comparable tothe ND filter 17 according to the first embodiment can be formed just bypreparing the four filter plates 37 having the same transmittance, andtherefore a production cost of the ND filter can be cut down.

FIG. 13 shows a second modification of the ND filter 17. An ND filter 39of this modification is made up of two filter plates 41 each comprisingtwo filter elements 40 a, 40 b which have different transmittances. Theupper filter element 40 a has transmittance of 40%, and the lower filterelement 40 b has transmittance of 16%.

A front filter plate 41 a is formed to have a vertical dimension smallerthan that of a rear filter plate 41 by a width β. An upper area of thefront filter plate 41 a corresponding to a vertical width 2β is formedby the upper filter element 40 a, and the remaining lower area of thefront filter plate 41 a is formed by the lower filter element 40 b.Likewise, an upper area of the rear filter plate 41 b corresponding to avertical width 2β is formed by the upper filter element 40 a, and theremaining lower area of the rear filter plate 41 b is formed by thelower filter element 40 b. Those front and rear filter plates 41 a, 41 bare arranged such that their lower edges are flush with each other (seeFIG. 13).

With the above arrangement, a portion 42 a of the upper filter element40 a of the rear filter plate 41 b which is not overlapped with thefront filter plate 41 a, a portion 42 b in which the upper filterelement 40 a of the rear filter plate 41 b and the upper filter element40 a of the front filter plate 41 a are overlapped with each other, anda portion 42 c in which the lower filter element 40 b of the rear filterplate 41 b and the upper filter element 40 a of the front filter plate41 a are overlapped with each other, each have a vertical width β. Also,a portion 42 d in which the lower filter element 40 b of the rear filterplate 41 b and the lower filter element 40 b of the front filter plate41 a are overlapped with each other has a vertical width 3β. Thus theportions 42 a, 42 b, 42 c and 42 d have respectively the same dimensionsas the filter elements 21 a, 21 b, 21 c and 21 d of the ND filter 17 inthe exposure control mechanism 1 according to the first embodiment.

The uppermost portion 42 a of the ND filter 39 (i.e., the portioncovering the vertical width β from an upper edge) is formed by only theupper filter element 40 a of the rear filter plate 41 b and hastransmittance of 40%. The portion 42 b positioned below the uppermostportion 42 a and covering the vertical width β is formed by the upperfilter element 40 a of the rear filter plate 41 b and the upper filterelement 40 a of the front filter plate 41 a overlapping with each other,and has transmittance of 16% (40%×40%). The portion 42 c positionedbelow the portion 42 b and covering the vertical width β is formed bythe lower filter element 40 b of the rear filter plate 41 b and theupper filter element 40 a of the front filter plate 41 a overlappingwith each other, and has transmittance of 6.4% (16%×40%). The portion 42d positioned below the portion 42 c and covering the vertical width 3βis formed by the lower filter element 40 b of the rear filter plate 41 band the lower filter element 40 b of the front filter plate 41 aoverlapping with each other, and has transmittance of 2.56% (16%×40%).Thus the portions 42 a, 42 b, 42 c and 42 d also have respectivelyalmost the same transmittances as the filter elements 21 a, 21 b, 21 cand 21 d of the ND filter 17 in the exposure control mechanism 1according to the first embodiment.

With the ND filter 39 of this modification, an ND filter comparable tothe ND filter 17 according to the first embodiment can be formed just bypreparing the two filter plates 41 a, 41 b each of which is made up oftwo filter elements 40 a, 40 b having different transmittances, andtherefore a production cost of the ND filter can be cut down. Inaddition, an optically superior ND filter can be provided withoutincreasing a filter thickness because the thickness of the ND filter 39is held not so large as that of the ND filter 36 of the above firstmodification.

In the above-described exposure control mechanism according to each ofthe above embodiments, the rotating arm(s) is coupled to the diaphragmblades and the ND filter holding member by providing the connecting pinson the side of the rotating arm and forming the connecting slots on theside of the diaphragm blades and the ND filter holding member. However,the image pickup apparatus of the present invention is not limited tothe illustrated embodiments. Alternatively, the connecting slots may beformed on the side of the rotating arm, and the connecting pins may beprovided on the side of the diaphragm blades and the ND filter holdingmember.

Also, the driving mechanism(s) for the diaphragm blades and the NDfilter holding member is not limited to the use of a motor, but maycomprise a rack and pinion unit or a linear motor it should beunderstood that any shapes and structures of the respective componentsin each of the above embodiments are illustrated merely by way ofexamples for implementing the present invention, and the aboveembodiments are not to be construed in a sense limiting the technicalscope of the present invention.

As is apparent from the above description, the present inventionprovides the following advantages. According to a first aspect, an imagepickup apparatus comprises an exposure control mechanism for adjustingthe quantity of light flux entering a shooting lens system, the exposurecontrol mechanism comprising a diaphragm made up of diaphragm bladesmovable on a plane perpendicular to an optical axis in oppositedirections to define a diaphragm aperture, and an ND filter made up ofat least two ND filter elements having different transmittances, whereinwhen the diaphragm blades are displaced from an aperture open state in adirection to restrict the quantity of transmitting light, an aperturearea is restricted by the diaphragm blades from the open state to apredetermined aperture area, and thereafter the ND filter is advancedinto the diaphragm aperture successively from one of the ND filterelements having the highest transmittance while the predeterminedaperture area is maintained. Even with an image pickup device having asmaller picture size and a shorter pixel pitch, therefore, it ispossible to reduce the effect of diffraction caused by a small apertureand to improve image quality. As a result, an image pickup device havinga small picture size and a short pixel pitch can be employed with lessdeterioration of image quality.

According to a second aspect of the present invention, the light fluxentering the shooting lens system is recorded on a recording mediumafter the quantity of the light flux has been adjusted by the exposurecontrol mechanism. Therefore, a high-quality image can be recorded withless deterioration of image quality.

According to a third aspect of the present invention, the exposurecontrol mechanism includes a first driving mechanism for driving thediaphragm blades and a second driving mechanism for driving the NDfilter. Therefore, various control of an optical system can be performedwith the diaphragm blades and the ND filter in an independent manner.

According to a fourth aspect of the present invention, the exposurecontrol mechanism includes one driving mechanism and a rotating platedriven by the driving mechanism, which cooperatively perform such aninterlock motion that the aperture area is restricted by the diaphragmblades from the open state to the predetermined aperture area, andthereafter the ND filter is advanced into the diaphragm aperturesuccessively from one of the ND filter elements having the highesttransmittance while the predetermined aperture area is maintained.Therefore, the two diaphragm blades and the ND filter can be moved byone driving mechanism at the same time over respective predeterminedranges.

According to a fifth aspect of the present invention, the ND filter isformed by placing a plurality of filter plates in adjacently overlappedrelation, the filter plates having the same transmittance but beingdifferent in size. Therefore, the ND filter can be formed by using thefilter plates having the same transmittance, and a production cost ofthe ND filter can be cut down.

According to a sixth aspect of the present invention, the ND filter isformed by placing a plurality of filter plates in adjacently overlappedrelation, each of the filter plates comprising at least two filterelements being different in transmittance and size. It is thereforepossible to reduce the number of filter plates used, lessen thethickness of the ND filter, and to cut down a production cost of the NDfilter.

According to a seventh aspect of the present invention, the ND filter isadvanced into the diaphragm aperture before the diaphragm aperturereaches the predetermined aperture area. Therefore, the so-called deadzone can be produced in which the quantity of light is not changedregardless of driving of the exposure control mechanism. As a result,various control of the optical system in the image pickup apparatus canbe performed with ease.

1. A method of controlling an exposure control mechanism of an imagepickup apparatus, comprising the steps of: determining a luminance levelof an image signal entering said image pickup apparatus; controllingmovement of a first diaphragm element, a second diaphragm element and anND filter unit having a plurality of filter elements, based on thedetermined luminance level; and calculating a total movement amount tobe performed in said controlling step, wherein said calculating stepcomprises: calculating an error amount based on a predetermined targetluminance level and the determined luminance level; and calculating thetotal movement amount based on the error amount and a previous movementamount; setting a control range and movement amount of said first andsecond diaphragms based on the calculated total movement amount; andsetting a control range and movement amount of said ND filter unit basedon the calculated total movement amount and the set movement amount ofsaid first and second diaphragms.
 2. A controlling apparatus for anexposure control mechanism of an image pickup apparatus, comprising: acamera signal processing unit adapted to determine a luminance level ofan image signal entering said image pickup apparatus; and a CPU adaptedto control movement of a first diaphragm element, a second diaphragmelement and an ND filter unit having a plurality of filter elements,based on the determined luminance level, wherein said CPU calculates atotal movement amount of said movement by calculating an error amountbased on a predetermined target luminance level and the determinedluminance level, and by calculating the total movement amount based onthe error amount and a previous movement amount, and said CPU sets acontrol range and movement amount of said first and second diaphragmsbased on the calculated total movement amount; and sets a control rangeand movement amount of said ND filter unit based on the total movementamount and the set movement amount of said first and second diaphragms.3. The controlling apparatus for an exposure control mechanism accordingto claim 2, wherein said first diaphragm element is positioned on anobject side and said ND filter unit is positioned on an image side, saidsecond diaphragm element being between said first diaphragm element andsaid ND filter unit.
 4. The controlling apparatus for an exposurecontrol mechanism according to claim 2, wherein a transmittance for oneof said plurality of filter elements differing from a transmittance foranother of said plurality of filter elements.
 5. The controllingapparatus for an exposure control mechanism according to claim 2,wherein a width for one of said plurality of filter elements is threetimes that of another of said plurality of filter elements.